Photovoltaic cells, also called solar cells, are devices that create electricity from light. The most common type is made from silicon in a process similar to the way computer chips are made and requires large expensive factories.One possible alternative to silicon cells is dye-sensitized cells, which are less efficient, but are far less expensive to manufacture. The dye absorbs light and transfers the excited electrons to the titanium dioxide. The titanium dioxide semiconductor material separates the charge. The redox couple completes the circuit. In this lab, we will use raspberry juice to construct a simple dye-sensitized solar cell and measure the electricity the cell produces.

Safety

Wear eye protection

Chemical gloves recommended

Procedure

Step 1. Identify the conducting side of a tin oxide-coated piece of glass by using a multimeter to measure resistance. The conducting side will have a resistance of 20-30 ohms.

Step 2. With the conducting side up, tape the glass on three sides to the center of a spill tray using one thickness of tape. Wipe off any fingerprints or oils using a tissue wet with ethanol.
Opposite sides of tape will serve as a spacer (see below) so the tape should be flat and not wrinkled. The third side of tape gives an uncoated portion where an aligator clip will be connected

Step 3. Add a small amount of titanium dioxide paste and quickly spread by pushing down and across with a microscope slide before the paste dries. The tape serves as a 40-50 micrometer spacer to control the thickness of the titanium dioxide layer if you push down.

Step 4. Carefully remove the tape without scratching the TiO2 coating. Leave the removed tape in a spill tray for disposal.

Step 5. Heat the glass on a hotplate in a hood for 10-20 minutes. The surface turns brown as the organic solvent and surfactant dries and burns off to produce a white or green sintered titanium dioxide coating. (Note: this requires a plate that gets quite hot.) Allow the glass to slowly cool by turning off the hotplate. The sample will look quite similar before and after heating; you only know it is done if you have observed the darkening stage along the way.

Step 6. Immerse the coating in a source of anthocyanins, such as raspberry juice. The raspberry juice may be obtained from frozen raspberries. (Blackberries, pomegranate seeds, and Bing cherries can also be used.) The white TiO2 will change color as the dye is absorbed and complexed to the Ti(IV).

Step 7. Rinse gently with water to remove any berry solids and then with ethanol to remove water from the porous TiO2. The ethanol should have evaporated before the cell is assembled.

Step 8. Pass a second piece of tin oxide glass, conducting side down, through a candle flame to coat the conducting side with carbon (soot). For best results, pass the glass piece quickly and repeatedly through the middle part of the flame.

Step 9. Wipe off the carbon along the perimeter of three sides of the carbon-coated glass plate using a dry cotton swab.

Step 10. Assemble the two glass plates with coated sides together, but offset so that uncoated glass extends beyond the sandwich. Do not rub or slide the plates. Clamp the plates together with binder clips.

Step 11. Add a drop of a triiodide solution to opposite edges of the plate. Capillary action will cause the KI3 solution to travel between the two plates. (The KI3 electrolyte solution consists of 0.5 M KI and 0.05 M I2 in anhydrous ethylene glycol.) The solution can corrode the aligator clips in the next step so wipe off an excess.

Properties

Connect a multimeter using an alligator clip to each plate (the negative electrode is the TiO2 coated glass and the positive electrode is the carbon coated glass).

Test the current and voltage produced by solar illumination, or...

Test the current and voltage produced by illumination from an overhead projector.

Did your solar cell work? Include the current and voltage (with units) produced by your solar cell in your conclusions. How much power is produced? (energy/time = volts x amps = watts)

What area of solar cell would be needed to produce 1 watt? (Assume the voltage produced is constant and that the current would be proportional to the area of the solar cell.)

Gather together all the cells you and your classmates made. How would you assemble them together to produce a maximum voltage? What about a maximum current?

What is the function of each part of the solar cell you built? One way to answer this question is to follow the path of an electron through the complete circuit.

How could you improve the efficiency of your solar cell?

Materials

A kit that contains the supplies (conductive glass, nanocrystalline TiO2, binder clips, KI3 electrolyte, manual, etc.) to create five titanium dioxide raspberry solar cells can be ordered from the Institute for Chemical Education. The kit contains enough nanocrystalline titanium dioxide to be used many times.

Preparation of TiO2 paste

-Grind about 0.5 gram of nanocrystalline titanium dioxide (TiO2) in a mortar and pestle with a few drops of very dilute acetic acid. Alternate grinding and addition of a few drops of very dilute acetic acid until you obtain a colloidal suspension with a smooth consistency, somewhat like latex paint. A toothpaste-like consistency is too thick. Also mix in a drop of clear dishwashing detergent as a surfactant. This quantity of TiO2 is enough for several solar cells.

For easier distribution, transfer the TiO2 paste to a syringe. Wrap the end of the syringe with parafilm to keep the paste from drying out when not in use. (If the paste dries out, the titanium dioxide will need more water. See previous step.) Using the syringe significantly shortens the class working time, makes for easier clean-up, and gives paste of proper consistency that will last for more than a lab period.

Frozen raspberries. The anthocyanin used as the dye must complex with the titanium(IV). Testing a variety of red or blue colored plant material is a possible source for inquiry-based extensions. Blackberries, raspberries, pomegranate, and bing cherries work. Strawberries and red grapes do not work. Because of general availability and the intense color we use thawed frozen raspberries pulverized in a blender; the mixture can refrozen and thawed many times.